Circuits for sound reproduction



Oct. 18, 1966 c s 3,280,258

CIRCUITS FOR SOUND REPRODUCTION Filed June 28, 1963 2 Sheets-Sheet 1 G445 5 CZ/Qf/S INVENTOR.

Oct. 18, 1966 G. B. CURTIS 3,280,258

CIRCUITS FOR SOUND REPRODUCTION Filed June 28, 1963 2 Sheets-Sheet United States Patent 3,280,258 CIRCUITS FOR SOUND REPRODUCTION Gale B. Curtis, 13532 Diamond Head Drive, Tustin, Calif. Filed June 28, 1963, Ser. No. 291,368 9 Claims. (Cl. 179-1) This invention relates to electrical apparatus for sound reproduction, and more particularly to a stereophonic sound system.

In many stereophonic sound reproduction systems, only two spaced speakers, are employed. The input to each such speaker is varied in a manner to give the impression to a listener that several different sounds that are heard emanate from sound sources located at different corresponding positions at and/ or intermediate the speak- 'ers.

Unfortunately, the simulation of sound sources at different positions by only two speakers is most successful only when a listener moves to a position precisely the same distance from each of the speakers. To a listener located in a position much closer to one speaker than to the other, most all sounds seem to emanate from exactly the same place, i.e. from the position of the nearest speaker.

' In the past, the problem of creating a stercop'honic effect for listeners located much closer to one of two speakers than to the other has been partially solved by employing a third speaker positioned about mid-way between the other two. A signal is then fed to the center speaker which is a fraction of the sum of the signals fed to the other two speakers. This prior art three speaker system has improved the stereop-hon-i-c effect for listeners not centrally located to a considerable extent. The addition of the central speaker provides this improvement because its addition makes it more difiicult for a listener to move very close to one speaker without being near another one.

Although for both central and noncentral listeners to a reproduction through a conventional three speaker system a stereophonic effect is discernible, all recorded sounds so reproduced always seem to emanate more from positions midway between the end speakers than they do to a centrally located listener listening to a reproduction of the same two channel recording on the above-described two speaker system.

The above-described and other disadvantages of the prior art are overcome by employing several speakers and by subtracting about one-half of the input to a center speaker or an equivalent thereof from the inputs to at least the two speakers at each end of a row of speakers. The theory of using this arrangement is that the existence of a center speaker is only a nuisance to a centrally located listener and its effect should be nullified for such a listener, if possible, without nullifying the stereophonic etfect'retaining function of a center speaker for a listener not centrally located. According to the present invention, this is accomplished in that one-half of the center speaker sound is transmitted from each end of the row of speakers in phase opposition. It is well known that out of phase sound waves of equal amplitude will cancel out. Thus, for the central listener located some distance from the speaker, the sound transmitted by the center speaker is effectively muffled.

Notwithstanding the foregoing, the improved stereoph-onic effect produced by the center speaker in accordance with the present invention for a listener not centrally located is maintained in that the sum of the subtracted sounds transmitted by the speakers at each end of a row is less than that transmitted by the center speaker at noncentral locations. Further, as before, a fraction of the sum of two stereophonic signals is introduced to the center speaker in accordance with the system of the present invention.

3 ,280,258 Patented Oct. 18, 1966 Although the theory of operation of the present invention has not gone through a more rigorous, quantitative or mathematical analysis than the above qualitative analysis, the analysis set forth herein has proven to be correct in that an actual reduction to practice has revealed that the present invention retains not only the two speaker effect on a central listener of simulating sound sources throughout the entire distance from the speaker at one end of a row to the one at the other end of the same row, but also the three speaker function of retaining a stereophonic effect for a listener not centrally located.

The above-described and other advantages of the present invention will be better understood from the following description when considered in connection with the accompanying drawings.

In the drawings, which are to be regarded as merely illustrative FIG. 1 is a block diagram of one embodiment of the invention;

FIG. 2 is a schematic diagram of an embodiment of the invention shown in FIG. 1;

FIG. 3 is a block diagram of another embodiment of the present invention.

In FIG. 1, a source of left and right stereophonic sig nals are provided on output leads 10 and 11 respectively by a source 12. A fraction of the sum of the output signals on leads 10 and 11 is produced at an output lead 13 from the adder 14. This fraction of the sum of the signals on leads 10 and 11 is impressed on a center speaker 15. One-half of the signal impressed upon the center speaker 15 is subtracted from the signal appearing on lead 10 by a subtractor 16. The output of subtractor 16 is then impressed upon a left hand speaker 17 by a lead 18. Similarly, one-half of the output of adder 14 is subtracted from the signal appearing on lead 11 by a subtractor 19. The output of subtractor 19 is then impressed upon a right hand speaker 20 via a lead 21.

One specific embodiment of the invention illustrated generally in FIG. 1 is shown in FIG. 2. In addition to source 12, leads 1i) and 11, and speakers 15, 17 and 2t), autotransformers T1 and T2 are shown in FIG. 2 with two variable resistors R1 and R2 which are ganged as indicated at 22.

Autotransformer T1 is connected from lead 10 to lead 11. Autotransformer T1 is provided with a center tap 23. Resistor R1 is connected from center tap 23 to resistor R2. Resistor R2 is connected from resistor R1 to ground. Resistors R1 and R2 are thus provided with a mutual junction at 24. Transformer T2 is connected from junction 24 to ground. Transformer T2 is similarly provided with a center tap at 25.

One side of speaker 20 is connected to lead 11. The other side of speaker 20 is connected to center tap 25 of transformer T2. One side of speaker 15 is connected from junction 24 between resistors R1 and R2 to ground. Speaker 17 is connected from center tap 25 of transformer T2 to lead 10.

It will be appreciated that the mean sum of the signals appearing on leads 10 and 11 is taken at center tap 23 of transformer T1. A fraction of the signal appearing at center tap 23 is taken from junction 24 of resistors R1 and R2. This signal is impressed upon center speaker 15. One-half of this signal is taken at center tap 24 of traii sformer T2 and subtracted from the signals appearing at leads 10 and 11 and thereafter respectively impressed upon left and right hand speakers 17 and 20.

The circuits appearing between lead 10 and ground and between lead 11 and ground have constant impedances when resistors R1 and R2 are ganged at 22 in a manner to maintain the resistance of resistor R1 equal to where Z is the impedance of each. speaker and R is the resistance of resistor R2.

Another embodiment of the present invention is shown in FIG. 3 including a source 36 of stereophonic signals which appear upon leads 37, 38 and 39 provided in general for a left hand speaker 40, a center speaker 41, and a right hand speaker 42, respectively. Before these s1g nals are impressed upon these three speakers, they are modified by a network indicated at 43. Network 43 includes an adder 44 for producing an output signal on a lead 45 proportional to a fraction of the sum of the signals appearing on leads 38 and 39. An adder at 46 is also provided for producing an output signal upon a lead 47 which is a fraction of the sum of the input signals appearing at leads 37 and 38. Adder 44 is connected from leads 38 and 39 to a right intermediate speaker 48. Adder 46 is connected from leads 37 and 38 to a left intermediate speaker 49.

Network 43 also employs three subtractors 50, 51 and 52. Subtractor 50 subtracts one-half of the output signal of adder 44 from the signal appearing upon lead 39. Subtractor 50 is thus connected from adder 44 and from lead 39 to right hand speaker 42.

Subtractor 51 subtracts one-half of the output signals of both adders 44 and 46 from the signal appearing upon lead 38 and impresses the same upon center speaker 41.

Subtractor 52 subtracts one-half of the output signal of adder 46 from the signal appearing upon lead 37 and impresses the same upon left hand speaker 40.

The embodiment of the invention shown in FIG. 3 may be similarly employed for a multichannel stereophonic sound system for any number of channels.

In this regard, it is to be noted that although all circuits disclosed herein are designed for speakers of identical impedances, other circuits could easily be designed for speakers of different impedances.

From the foregoing, it will be appreciated that for a centrally located listener, the nuisance of a center speaker is obviated in that the effect of the same is nullified by subtracting one-half of the center speaker signal from the signals impressed upon the left and right hand speakers. The value of centrally located speakers is, however, retained for an off center listener since the effect of the center speaker is not nullified for a listener not centrally located. Note will be taken that when a listener not centrally located is positioned between a center speaker and only one end speaker, the center speaker nullifying effect of the end speakers is reduced in that such a listener is located a substantial distance from one end speaker. That is, he may be located closer to one end speaker, but he will be located a substantial distance farther away from the other end speaker.

From the foregoing, it will also be appreciated that any of the specific circuits of the present invention shown in the drawings may be employed by the use of only a few inexpensive component parts for either two or three channel stereophonic sound systems, or for multichannel stereophonic sound systems employing more than three channels.

Although only a few specific embodiments of the present invention have been shown and described herein, many changes and modifications will of course suggest themselves to those skilled in the art. These embodiments have been selected for the purpose of illustration only. The present invention should therefore not be limited to the embodiments so selected, the true scope of the invention being defined only in the appended claims.

What is claimed is:

1. In a stereophonic sound system, the combination comprising: a source of two stereophonic audio signals; first and second speakers; intermediate speaker means; means to introduce the same fraction of each of said audio signals to said intermediate speaker means; means to introduce one of said audio signals to said first speaker less a second fraction of the input to said intermediate speaker means; and means to introduce the other of said audio signals to said second speaker less said second fraction of the input to said intermediate speaker means.

2. In a stereophonic sound system, the combination comprising: a source of two stereophonic audio signals; first and second speakers; intermediate speaker means; means to introduce the same fraction of each of said audio signals to said intermediate speaker means; means to introduce a signal to said first speaker equal to one of said audio signals minus one-half of the input to said intermediate speaker means; and means to introduce a signal to said second speaker equal to the other of said audio signals minus one-half of the input to said intermediate speaker means.

3. The method of distributing sound energy comprising the steps of: transmitting a sound from a central location of a magnitude equal to a fraction of the sum of the magnitudes of two stereophonic sounds; simultaneously transmitting a sound from one 'side of said central location of a magnitude equal to that of one of said stereophonic sounds minus a fraction of the sound transmitted from said central location; and simultaneously transmit ting a sound from the other side of said central location equal to that of the other of said stereophonic sounds minus a fraction of the sound transmitted from said central location.

4. In a stereophonic sound system, the combination comprising: means to produce a first stereophonic audio signal; means to produce a second stereophonic audio signal; adder means responsive to said audio signals for producing a central output signal proportional to the sum of said audio signals; first subtractor means responsive to said first audio signal and to said central output signal for producing a first output signal proportional to a difference therebetween; second subtractor means responsive to said second audio signal and to said central output signal for producing a second output signal proportional to a difference therebetween; and speaker means responsive to the output of each of said adder and subtractor means.

5. In a stereophonic sound system, the combination comprising: a source for producing two different stereophonic signals on two different corresponding leads relative to a point of reference potential; .a first center tapped autotransformer connected between said leads; a second center tapped autotransformer connected with the center tap on said first autotransformer to said point of reference potential; a first speaker connected from one of said leads to the center tap on said second autotransformer; a second speaker connected from the other of said leads to said second autotransformer center tap; and a third speaker connected from said first autotransformer center tap to said point of reference potential.

6. In a stereophonic sound system, the combination comprising: a source for producing two different stereophonic signals on two different corresponding leads relative to a point of reference potential; a first center tapped autotransformer connected between said leads; first and second resistors respectively connected serially from said first autotransformer center tap to said point of reference potential; 21 second center tappedautotransformer and a central speaker connected in parallel from the mutual junction of said resistors to said point of reference potential; a first side speaker connected from one of said leads to said second autotransformer center tap; and a second junction of said resistors to said point of reference poten tial; a first side speaker connected from one of said leads to said second autotransformer center tap; and a second side speaker connected from the other of said leads to said second autotransformer center tap, said first resistor having a resistance equal to (Z/2R) (2Z-R) where Z is equal to the impedance of each of said speakers and R is equal to the resistance of said second resistor.

8. In a stereophonic sound system, the combination comprising: a source :for pnodncing two different stereophonic signals on two diflferent corresponding leads rel-ative to a point of reference potential; a first center tapped autotransformer connected between said leads; first and second resistors respectively connected serially from said first :autotransformer center tap to said point of reference potential; a second center tapped autotransfo-rmer and a central speaker connected in parallel from the mutual junction of said resistors to said point of reference potential; a first side speaker connected from one of said leads to said second a-utotransformer center tap; a second side speaker connected from the other of said leads to .said second transformer center tap, said resistors being adjustable; and means to adjust said resistors so that the resistance of said first resistor is equal to where Z is the impedance of each of said speakers and R is the resistance of said second resistor.

9. In a stereophonic sound system having several channels, the combination comprising: a source for producing a plurality of more than two stereophonic audio signals on a difierent corresponding lead for each; a row of main speakers including a speaker for each of said audio signals; an auxiliary speaker adapted to be disposed between each adjacent pair of main speakers; mean for introducing an input signal to each of said auxiliary speakers equal to a fraction of the sum of the magnitudes of the audio signals corresponding to the two main speakers immediately adjacent thereto; and means for introdncing a signal to each main speaker equal to the magnitude of each of said audio signals corresponding thereto minus one-half the magnitude of the sum of the input signals to the two auxiliary speakers immediately adjacent thereto, the main speaker at each end of said row thus having an input signal equal in magnitude to that of its said corresponding audio signal minus one half the magnitude of the input to the single auxiliary speaker adjacent thereto.

No references cited.

KATHLEEN H. CLAFFY, Primary Examiner.

S. J. BOR, Assistant Examiner. 

1. IN A STEREOPHONIC SOUND SYSTEM, THE COMBINATION COMPRISING: A SOURCE OF TWO STEREOPHONIC AUDIO SIGNALS; FIRST AND SECOND SPEAKERS; INTERMEDIATE SPEAKER MEANS; MEANS TO INTRODUCE THE SAME FRACTION OF EACH OF SAID AUDIO SIGNALS TO SAID INTERMEDIATE SPEAKER MEANS; MEANS TO INTRODUCE ONE OF SAID AUDIO SIGNALS TO SAID FIRST SPEAKER LESS A SECOND FRACTION OF THE INPUT TO SAID INTERMEDIATE SPEAKER MEANS; AND MEANS TO INTRODUCE THE OTHER OF SAID AUDIO SIGNALS TO SAID SECOND SPEAKER LESS SAID SECOND FRACTION OF THE INPUT TO SAID INTERMEDIATE SPEAKER MEANS. 